1. Field of the Invention
The present invention relates to a strain sensor detecting displacement, impact, stress, strain, deformation, and/or distortion on materials, objects, matters, and/or substances, and in particular to a strain sensor using carbon nanotubes.
The present application claims priority on Japanese Patent Application No. 2011-249172, the entire content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, strain sensors are designed to detect strain based on resistance variations, e.g. resistances of resistors which are varied due to strain (e.g. expansion and contraction) applied thereto. Generally, metals or semiconductors have been used for resistors. However, metals or semiconductors have limitations when applied to strain sensors because they may produce a small amount of strain due to reversible expansion and contraction.
Considering this drawback, various devices using carbon nanotubes (CNTs) as resistors have been developed and disclosed in various documents such as Patent Literatures 1 to 4. Patent Literatures 1-3, which were written by the same applicant but filed in different ways, are each directed to smart materials; strain sensing and stress determination by means of nanotube sensing systems, composites, and devices using carbon nanotubes that are capable of detecting displacement, impact, stress, and/or strain in materials. Patent Literature 4 discloses an expansion/contraction device using carbon nanotubes. Additionally, Non-Patent Literature 1 is an article entitled “Growth and Application of Ultra-Long Multi-Walled Carbon Nanotube”, which focuses on an easy and efficient synthesis method of vertically-aligned ultra-long multi-walled nanotubes using iron chloride powder.
The foregoing devices uses CNT films made of a plurality of CNTs oriented in a predetermined direction. It is believed that CNT films are applicable to sensors detecting a high strain because CNT films can be largely expanded or contracted in a perpendicular direction to the orientation direction of CNTs. However, the foregoing devices are produced using CNT films which are exposed on their surfaces. This may cause abnormality in resistance variations of CNTs (serving as resistors), for example, when CNT films are unexpectedly damaged or broken due to abrupt contact with materials, when foreign matters unexpectedly enter into gaps formed between CNTs, and/or when moisture or floating gas is applied to CNTs. This may degrade durability (or machine life) in sensing functionality.